Rin is a demon, but he's a slacker. Satan's constantly breathing down his neck, telling him to possess people, influence politicians etc, but he's having none of that. He'd rather play video games all day and hang out with you, who so happens to be an human. Rin is an absolute troublemaker, which the other demons criticise him for. He met you on a mission when he was struggling to possess an old lady. You laughed at him. Now you guys are best friends even though it's illegal.